Fuel Injector Issue

07-31-2012, 12:44 PM
I recently had a problem with my '90 Celebrity V6 3.1L engine. It behaved a lot like a fuel pump/fuel filter issue. At first it wast hard to start, and it would lose power on hard accelerations. The next day it stopped completely and would not re-start start. It still would not restart after sitting for a couple hours. However, it started right away after being towed home.

After changing the fuel filter it continued to drop out on hard accelerations and at even lighter power, but coming right back if the trottle was backed off. It was a apparent there was a "sweet spot" where you'd get the best power, although this was not consistent from day to day.

Sometimes it wouldn't start at all after a drive and sitting ~ 1 hr, but then start right up many hours later.

I also noticed that it misfired a bit under idle, and there was a slight engine hesitation when applying the accelerator.

The fuel pressure test OK, and the fuel pressure regulator was within specifications. It would spit and even back fire if you ran > 1/2 throttle in the garage. The TPS checked out OK as did the EGR and IAT sensor.

An oscilloscope of the 1 & 6 injector waveforms showed very errattic signals at the heavy trottle position.

I was very puzzled at the cause.

Finally it stopped completely and could not be restarted at all. I eventually checked the resistance of the two injector banks, and one tested at 4.4 ohms while the other was 1.0 ohms. Each bank is the parallel combination of three injectors, and the normal resistance is ~ 4 ohms. I removed the upper intake manifold and checked the resistance of each injector, and found that the injector on cylinder 2 was only 1.1 ohm. All others were ~12 ohms as expected.

I removed the injector and decided to test its operation, applying +12 V from the battery. It "clicked" as expected and seemed to work fine. I rechecked the resistance and found that it has increased to 8-9 ohms.

Of course one bad injector would not normally cause the engine to stop running, but in this case the low resistance was draining all the current from the ECM, preventing any of the injectors from opening. The computer normally delivers 6 amps of current during the pulse, but the faulty injector was drawing 12 amps alone. Since the computer was never designed to deliver so much current (17 amps total), the voltage was pulled down and none of the injectors opened.

I believe it was previously able run at idle because the resistance was not as low as I found, because pulse width was relatively low, and the computer could temporarily handle this extra drain. However, at high trottle the pulses are wider and the ECM was unable to supply enough current to deliver the necessary fuel. The times it would not start were probably caused by the injector resistance becoming small due to some temperature dependancy, but it would run once that resistance increased and the current drawn was less than the maximum the ECM could handle.

I replaced the one faulty injector with a new one, and it has been running fine since.

07-31-2012, 02:29 PM
Good diagnosis, not uncommon issue with the GM injectors from the that time. Just hard to get to for the resistance test.

Tech II
07-31-2012, 03:08 PM
Yes, this is what they call a multiport setup....1-3-5 are all fired at the same time by one quad driver....2-4-6 are all fired at the same time by another quad driver.......

So if one injector shorts out, the quad driver detects too much current and will shut down, thus killing 3 cylinders....

Joe W
08-03-2012, 11:55 PM
Thanks for your reply for your issue... this helps us all!!!

10-11-2012, 08:10 AM
Hay great info, thanks for sharing

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